A Comprehensive Study of Fractal Models for Permeabilities under Thermo-Hydro-Mechanical Interactions and Slip Effect in Porous Coal Rocks

The complex pore-fracture dual-medium structure in coal reservoirs plays an important role during methane extraction. However, it remains a challenge to elucidate the effects of fracture aperture variation and gas slip on gas transport under thermo-hydro-mechanical (THM) interactions. In this paper, fractal geometry theory was applied to characterize the microstructures of porous coal rocks, and this work developed a three-dimensional (3D) THM coupling model that incorporates fractureaperture variation and the slip effect. This model enables simultaneous analysis of the stress field, porosity, permeability, seepage field, and temperature field. Furthermore, we analyzed the evolution of the slip effect with microstructural parameters, including: (1) fractal dimensionfor tortuosity; (2) fracture roughness; and (3) fluctuation amplitude of fracture cross-section size. The effects of Langmuir adsorption parameters on continuous and slip flow permeabilities were also calculated. The present results indicate that the slip effect enhances permeability in both pore and fracture systems, with a more prominent influence on pores. The fractal dimension for tortuosity exerts the strongest inhibitory effect on slip-flow permeability, followed by the fluctuation amplitude of fracture cross-section size, while fracture roughness has the weakest influence. The Langmuir adsorption parameters exhibit differentiated properties of continuous and slip flow.